In the field of cantilever type drilling rigs a known design has an X-Y movable cantilever that is mounted on a pair of support rails that extend parallel to a side of the rig to allow for translation of the cantilever along the side of the rig. The cantilever is movable at right angles to these support rails to vary the overboard extension of the cantilever. For example the U.S. Pat. No. 6,729,804 describes a drilling rig having a jack-up deck supporting structure deck with a deck, a cantilever and a supporting cart disposed between the cantilever and the deck. The supporting cart slides along the side of the deck whilst the cantilever slides at right angles thereto on the supporting cart on four friction reducing bearings.
The U.S. Pat. No. 7,083,004 describes alternatives to an X-Y movable cantilever which are aimed at increasing the drilling envelope, i.e. the area outside the deck that can be reached with drilling or borehole equipment on the cantilever by moving the cantilever into extended position. This known offshore drilling system has a rig with a deck, a cantilever and a supporting cart arranged between the cantilever and the deck. The supporting cart is pivotal about a vertical axis swivel arranged on the deck of the rig, whilst the cantilever can slide in its longitudinal direction on the supporting cart. The swivel is disclosed to be arranged at a fixed position on the deck or movable along a rail that extends over the centre of the deck, parallel to the overboarding side of the deck. In these alternative designs the cantilever is mounted on the deck so as to be movable in longitudinal direction of the cantilever, in rotational direction, and in combinations thereof.
The document US2010/0260555 also relates to an offshore drilling system with a rig having a deck and a cantilever, wherein the cantilever is mounted on the deck so as to be movable in longitudinal direction of the cantilever, in rotational direction, and in combinations thereof. Herein the rig has a pair of substantially parallel support rails extending along a side of the deck. The cantilever is provided with two parallel cantilever beams extending in longitudinal direction of the cantilever structure. A travelling and swilling assembly is arranged at each interposition of the two support rails and the two cantilever beams. Herein each assembly comprises an upper sliding block, a lower sliding block and a swivel member interconnecting the upper and lower sliding blocks, such that the assembly is moveable relatively on the relevant rails and cantilever beam to translate and/or rotate the cantilever with respect to the deck.
In cantilever type drilling rigs the weight of the cantilever and the loads to which the cantilever is subjected during drilling or borehole operations lead to enormous mechanical loads on the relevant parts of the system.